1. Field of the Invention
The present invention relates to a variable light signal delay apparatus for delaying a light signal and a variable light signal delay system using the same, and particularly relates to a variable light signal delay apparatus for varying a delay time of the light signal with a simple configuration and a variable light signal delay system using the apparatus.
2. Description of the Related Art
Through the rapid spread of ADSL (Asymmetric Digital Subscriber Line), FTTH (Fiber-to-the-Home), etc. in the access system, the practical implementation of Gigabit Ethernet (registered trademark), and so forth, an amount of information is rapidly increasing. For this reason, the importance and the demand for an increase in speed and capacity of the communication system are rising, and the optical network system is expected as the technology to realize a higher speed and a larger capacity of the communication system.
The optical communication is classified roughly into the burst type such as the transaction file type, or the like, the packet type used in the IP (Internet Protocol), and the intermittently continued bit stream type used in the telephone service, or the like.
For example, an address contained in a header of a light signal packet must be processed in the IP packet. Mainly the electric signal processing is applied nowadays after the light signal is converted into the electric signal, and therefore it is difficult to improve a performance of the communication system. Accordingly, the tremendous improvement in performance and the increase in speed can be expected if the signal processing can be applied to the light signal as it is. The light signal must be delayed in such optical signal processing, and thus delay elements, delay circuits, and the like are contained in the optical communication system.
FIG. 7 is a view showing a configuration of an optical delay apparatus in the related art. In FIG. 7, an optical coupler 1 causes an input light signal to branch off into two signals, and then outputs these signals. A delay optical fiber 2 is connected to one output end of the optical coupler 1.
An operation of such optical delay apparatus will be explained hereunder.
The light signal is input into the optical coupler 1. Then, the optical coupler 1 causes the input light signal to branch off into two signals that have a substantially same light intensity respectively. Then, the light signal being output from one output end of the optical coupler 1 passes through the delay optical fiber 2, and then is output therefrom. As a result, the light signal being output through the delay optical fiber 2 is delayed from the light signal being output the other output end of the optical coupler 1 by a time that corresponds to an optical path length of the delay optical fiber 2, and then is output therefrom.
Next, FIG. 8 is a view showing a configuration of a 2-input/2-output optical delay apparatus in the related art (see JP-A-6-237222, for example). In FIG. 8, a passive crossover switch 3 outputs the light signal being input from one input port via a delay optical fiber 4. An active optical switch 5 receives the light signal from the other input port and the passive crossover switch 3 respectively, and outputs the light signal from a desired output port by switching the path.
An operation of such optical delay apparatus will be explained hereunder.
The light signal being input from one input port is switched by the passive crossover switch 3 to pass through the delay optical fiber 4, and then is input into the active optical switch 5. Then, the active optical switch 5 switches the optical path to output the light signal from the desired output port. Also, the light signal being input from the other input port is output from the desired output port via the active optical switch 5. As a result, the light signal being input from one input port can be output with a delay.
Next, FIG. 9 is a view showing a configuration of an optical coupling/branching apparatus in packet units (see JP-A-2002-319896, for example). In this apparatus shown in FIG. 9, an optical delay element 6 for delaying the light signal is also provided. In FIG. 9, the input light signal is input into the optical delay element 6 and an optical label selector 7 respectively. Then, the optical label selector 7 operates an optical switch 8 based on the label affixed to the header of the packet. In the meanwhile, the optical delay element 6 delays the packet until an operation of the optical switch 8 is completed, and then outputs the packet to the optical switch 8. As a result, optical coupling/branching operations such as pulling-down and insertion in packet units can be carried out.
JP-A-6-237222 (Paragraph No. 0025, FIG. 6) and JP-A-2002-319896 (Paragraph Nos. 0024 and 0025, FIG. 2) are referred to as the related art.
However, in the related art shown in FIG. 7, for example, a structure in which the optical branch element (e.g., the optical coupler 1) and the optical delaying means (e.g., the delay optical fiber) are provided separately is used as a basic structure. Also, in the related arts shown in FIG. 8 and FIG. 9, a structure in which the optical switch and the optical delaying means (e.g., the delay optical fiber 4, the optical delay element 6) are provided separately is used as a basic structure. Therefore, the above apparatuses are developed to aim at functions that are executed on the premise that various processes including the optical switching, which is executed after a fixed delay is applied to the light signal, are carried out. As a result, such a problem existed that neither a function of varying a delay time of the light signal is provided nor a configuration that can respond flexibly to a system construction of the optical network system is provided. Also, such another problem existed that it is difficult for the above apparatuses to cope with the optical signal processing that must be executed in the optical communication system, the measuring equipment for the optical communication, and the like, for example, the high-level signal processing of varying a shift of the optical bit.